Bismillah.
Someone once told me that it's better to make du'a in the sajda (prostrating) when we are praying a sunnah prayer, rather than to make it during a mandatory prayer. Is this true? Or can we still make dua'a during a mandatory prayer as I often do? Is there a hadith that references this question?
Also, when saying subhana rabia al a'ala when doing the sajda, how many times are we meant to say it before making a du'a? Is one sufficient? Or is it three times?